1. Field of the Invention
The present invention relates to a rear projection display device, and particularly relates to a rear projection display device with reduced height and depth. When viewing from a front side of the rear projection display device, substantially only the screen can be seen.
2. Description of Prior Art
Various types of display devices have been widely adopted in ordinary life. Exemplary display devices include CRT (cathode ray tube) displays, rear projection displays, plasma displays, LCDs (liquid crystal display) and so on.
Recently, with the development of technology and increase of requirement, it has become a general tendency that a display device is designed to have a large-sized screen. Numerous attempts have been made to design such a display device with a large-sized screen. Among the various types of display devices, a CRT display with a screen size more than 40 inches (measured diagonally) become impractical because of the size and weight limitations of the CRT technology. The LCD display can be designed to have a screen size more than 50 inches, but the cost is very expensive. Currently, only the rear projection display and the plasma display are available at a reasonable cost while providing a screen size more than 50 inches. In comparison with a plasma display, a conventional rear projection display is large in volume. In addition, beneath the screen of the rear projection display, a significant amount of additional space is generally provided to accommodate an optical engine and a projection lens. Nevertheless, the rear projection display has an advantage of reduced cost over the plasma display, especially when a screen size larger than 60 inches is required.
FIG. 1 is a schematic view of the configuration of a conventional rear projection display 100 that is widely adopted. This rear projection display 100 includes a projection engine 140, a projection lens 130, a rear reflector 120 and a screen 110. The image produced by the projection engine 140 is transmitted through the projection lens 130, and then is reflected onto the screen 110 by the rear reflector 120. This conventional configuration leads to a significant depth of the display device. For example, for a 60-inch screen, the depth of the rear projection display 100 would be 24 inches. Further, since the projection engine 140 is disposed underneath the screen 110, a significant amount of additional space must be provided beneath the screen 110 so as to accommodate the projection engine 140 and the projection lens 130.
FIG. 2 is a schematic view of the configuration of an improved display 200 employs three reflectors 220, 230 and 240. An image light beam produced by a projection engine 260 is first transmitted through a projection lens 250, and then is sequentially reflected by the reflectors 240, 230 and 220 to project onto a screen 210. To enlarge the projection angle, one of the three reflectors 220, 230 and 240 is configured as an aspheric reflector. This improved rear projection display 200 has a reduced depth, which is achieved by reflecting the projected image light beam three times. In comparison with the display 100 in FIG. 1, the depth of this improved display 200 can be reduced to 12 inches for a 60-inch screen. However, this improved conventional display 200 also has a disadvantage that the cost is very high. This is caused by the exact adjustment of three-time reflection, the production of an aspheric reflector and the compensation for induced image distortion. In addition, the adoption of three-time reflection requires that the projection engine 260 be disposed at a position further below the screen 210 than that of the projection engine 140 in FIG. 1. This inevitably increases the height of the display 200. When viewed from a front side of the display device, the proportion of the screen 210 to the front side of the display device is even smaller than that of the configuration in FIG. 1.
FIG. 3 is a schematic view of the configuration of another improved conventional rear projection display 300 with a further reduced depth. This conventional rear projection display 300 employs a front small reflector 330 positioned below a screen 310, and a rear large reflector 320 in rear of the screen 310. An image light beam produced by the projection engine 340 is first reflected by the front small reflector 330, and then is reflected by the rear large reflector 320 toward the screen 310 for display. Particularly, the screen 310 is designed to include a plurality of angularly discriminating reflective elements 311. These reflective elements 311 are adapted to reflect light incident on the screen 310 from a first angle toward the rear reflector 320, and to allow light incident on the screen 310 from a second angle to be transmitted through the screen 310 for display. By employment of these particular reflective elements 311 on the screen 310 and the large rear reflector 320, this conventional display 300 can be made slim. Alternatively, to reduce the display depth, the screen 310 may be configured to reflect light of one polarity but transmit light of the other polarity. Polarized light from the projection engine 340 is first reflected off the screen 310 toward a polarization-rotating rear mirror, and then reflected off the rear mirror, which rotates the polarization 90 degrees so that the light can pass through the screen 310. However, both the above solutions require a particularly designed screen and the fine adjustment of component positions, which results in a high cost and thus is adverse to mass production at a low cost. Further, when viewed from a front side of the display device, the height of the display 300 is still considerable, since the projection engine 340 is positioned below the screen 310.
Accordingly, to overcome the above disadvantages in the prior art and satisfy the requirement for a compact display device, it is desired to provide an improved rear projection display device that has a reduced volume and therefore a low cost.